When you are using the Tableau's built-in map location that happens very late in Tableau's pipeline, after aggregation. So what's happening is that there are 7 marks for Island all stacked up on top of one another, not 1 mark with the sum of all 7. The two current ways to fix this are to a) do the correction in your data source or b) create a calculated dimension in Tableau (such as using CASE or IF/THEN statements) to use for your geography.

Tableau v10's new Custom Territories feature Coming Soon will enable you to do the grouping in Tableau and then do the geographic assignment.

In further testing there's a 9.2 solution for this, you can create a group of your geographic field where those 7 islands are batched together into a group, use that as a geography, and give that group the Island geography.